Mesopotamia was such a good area for growing crops because the rivers would flood and deposit nutrient and mineral-rich soil onto the dry land when it receded. Growing crops is a tricky business. Like other areas, Mesopotamia most likely had its fair share of pests, weeds, and other heavy vegetation that had to be taken care of before anything else could be done.
The floods would also destroy some crops if they were high enough or strong enough. Soft flooding is often needed for rice paddies - since the rice paddies are full of water with walkways of grass. Other plants - such as wheat and tobacco - need a drier and less likely to flood climate that will keep their roots from being uprooted.
